The present specification incorporates by reference U.S. Pat. No. 5,994,711, assigned to the assignee hereof.
The present invention relates to office equipment such as printers and copiers, and in particular relates to a system for registering the placement of multiple images on a sheet, such as to print images on both sides of a sheet.
Office equipment, such as printers and copiers, which place images based on digital data onto sheets, such as sheets of paper, are well known. More sophisticated types of office equipment are capable of placing images on both sides of a single sheet of paper, a feature often referred to as xe2x80x9cduplexing.xe2x80x9d A typical configuration of a duplexing printer (the word xe2x80x9cprinterxe2x80x9d including other types of equipment, such as digital copiers and facsimile machines) will include a marking device, meaning some hardware which places a desired image on a sheet, which is capable of printing only on one side of the sheet at a time. In order to print on both sides of the same sheet, it is necessary to feed a sheet through the marking device the first time, so the sheet can receive a first image on one side thereof, and then invert the sheet and re-feed it back into the marking device so that the marking device can place a second image on the other side of the sheet. Although the specific architectures of various office equipment on the market varies widely, the path (along with any associated sheet-handling hardware, such as belts or rollers and motors) by which a sheet which has been output by a the marking device is inverted and re-fed to the marking device can be generally referred to as a xe2x80x9cduplex path.xe2x80x9d In the market for office equipment having a duplex features, a common customer requirement is a precise registration between an image printed on one side of the sheet with the image printed on the other side. If a single sheet having a images on both sides a thereof is held up to the light, it is desirable that the margins of the two images, particularly if the images include text, be perfectly superimposed. There is therefore a need to provide a system by which the image placed on one side of a sheet by the marking device is registered with the image on the other side of the sheet.
U.S. Pat. No. 4,538,905 discloses a basic system in which a location of the side edge of a sheet being fed is precisely measured, and a position of an image placed on the sheet is thereby adjusted.
U.S. Pat. No. 5,994,711, which is incorporated by reference herein, describes a system, including an optical sensor, for the determining the position of a sheet being fed through a system, such as a copier. This precisely determined location is then used to control the marking device, such as an electrophotographic xe2x80x9claser printer,xe2x80x9d so that the image can be precisely placed on the sheet in response to the observed position of the sheet entering the marking device. The particular embodiment of this patent shows a preferred system for locating an edge of a sheet being fed, which can take into account of the distance of a sheet from an optical sensor when its position is being determined.
According to one aspect of the present invention, there is provided a printing apparatus, comprising a marking device, the marking device being capable of placing an image on a sheet fed therein and outputting the sheet; a feed path, for feeding a sheet from a sheet supply to the marking device; and a duplex path, by which a sheet output from the marking device is re-fed to the marking device. A first edge position detector outputs a first edge position signal relating to a location of a side edge of a sheet passing through the feed path, and a second edge position detector outputs a second edge position signal relating to a location of a side edge of a sheet passing through the duplex path. An image placement controller is associated with the marking device and adjusts placement of an image on the sheet in response to at least one of the first edge position signal and the second edge position signal.
According to another aspect of the present invention, there is provided a printing apparatus, comprising a first marking device, the marking device being capable of placing an image on a sheet fed therein, and outputting the sheet, and a second marking device, the marking device being capable of placing an image on a sheet fed therein, and outputting the sheet. A feed path is provided, for feeding a sheet from a sheet supply to the first marking device and then to the second marking device. A first edge position detector, for outputting a first edge position signal relating to a location of a side edge of a sheet passing through the feed path before the first marking device, and second edge position detector outputs a second edge position signal relating to a location of a side edge of a sheet passing through the feed path before the second marking device. An image placement controller is associated with at least one marking device, the image placement controller adjusting placement of an image on the sheet in response to at least one of the first edge position signal and the second edge position signal.